In the past procedures have been proposed for the conversion of fibers formed from acrylic polymers to a modified form possessing enhanced thermal stability. Such modification has generally been accomplished by heating the fibrous material in an oxygen containing atmosphere, such as ordinary air, at moderate temperatures for extended periods of time. The resulting product may be suitable for use as an intermediate in the formation of carbonized fibrous materials, or for direct utilization as a fire resistant fiber. U.S. Pat. Nos. 2,913,802 to Barnett and 3,285,696 to Tsunoda disclose processes for the conversion of fibers of acrylonitrile homopolymers or copolymers to a heat resistant form. Such prior art stabilization techniques have commonly been directed to batch operations employing acrylonitrile copolymers. Belgian Pat. No. 700,655 discloses a procedure whereby a continuous length of an acrylonitrile copolymer may be continuously subjected to a preoxidation treatment to produce essentially complete oxygen saturation while maintained in air at a temperature not exceeding 250.degree.C., e.g. three hours or more at 220.degree.C. Belgian Pat. No. 678,679 and French Pat. No. 1,471,993 disclose conducting the complete stabilization process in an inert atmosphere.
The stabilization of fibers of acrylonitrile homopolymers and copolymers in an oxygen containing atmosphere involves (1) an oxidative cross-linking reaction of adjoining molecules as well as (2) a cyclization reaction of pendant nitrile groups to a condensed dihydropyridine structure. While the reaction mechanism is complex and not readily explainable, it is believed that these two reactions occur concurrently according to the prior art, or are to some extent competing reactions.
The cyclization reaction is exothermic in nature and must be controlled if the fibrous configuration of the acrylic polymer undergoing stabilization is to be preserved. As indicated, prior art techniques have commonly overcome this difficulty by heating the fiber at moderate temperatures generally extended over many hours.
It is an object of the invention to provide an improved process for the flame-proofing of fibrous materials formed from acrylic polymers.
It is an object of the invention to provide an improved process for the thermal stabilization of fibrous materials formed from acrylic polymers.
It is an object of the invention to provide non-burning stabilized acrylic fibrous materials which exhibit an enhanced molecular structure.
It is an object of the invention to provide a process for the production of stabilized acrylic fibrous materials which exhibit an unusually high modulus.
It is another object of the invention to provide an improved stabilization process for fibrous materials formed from acrylic polymers which results in a product which is suitable for carbonization, or carbonization and graphitization.
It is another object of the invention to provide a process for the stabilization of a fibrous material formed from an acrylic polymer which in at least some of its embodiments may be conducted on a highly expeditious continuous basis.
It is a further object of the invention to provide a stabilized lightweight fibrous material which is capable of end use applications in the aerospace industry, and in numerous other industrial areas.
It is a further object of the invention to provide a process for the efficient conversion of acrylic fibrous materials to a non-burning form which effectively overcomes the difficulty normally presented by the critical exotherm upon the subjection of such starting materials to heat.
It is a further object of the invention to provide a process for converting a fibrous acrylic material to a stabilized form possessing essentially the identical fibrous configuration as the starting material.
These and other objects, as well as the scope, nature and utilization of the invention will be apparent from the following detailed description and appended claims.